PROJECT SUMMARY Ultraviolet (UV) light is the most prevalent environmental carcinogen, and UV light exposure is a major risk factor for melanoma. With chronic UV exposure, UV-induced DNA mutations accumulate in melanocytes within the epidermis and may disrupt tumor suppressors genes, leading to malignant transformation into melanoma. However, the majority of UV-induced mutations found in melanoma are passenger mutations that do not contribute to tumor development, but form neoantigens that have the potential to be targeted by the immune system. After decades of unsuccessful attempts to achieve immune destruction of melanoma, the clinical development of immune checkpoint inhibitors within the last decade finally improved the outlook for some patients with this traditionally devastating disease. The remarkable responses to immune checkpoint blockade (ICB) in melanoma have generated intense research interest in ICB. The immense UV-induced neoantigen load in melanoma provides ample targets for the immune system and may contribute to the relatively high rates of response in melanoma. However, a vast amount of overlap in neoantigen load exists between responders and nonresponders, suggesting that other anti-tumor mechanisms may exist. Interestingly, development of vitilitgo, which results from autoimmune destruction of melanocytes, has been correlated with favorable response in melanoma patients treated with ICB, suggesting that immune targeting of self-antigens shared by melanoma cells and normal melanocytes may also play a role. In the context of ICB, initial immune responses against melanoma neoantigens may produce epitope spreading resulting in an autoimmune response against melanocytic antigens that contributes to anti-tumor immunity. In Aim 1, this project will use genetically matched mouse models of melanoma with low or high neoantigen load to test the hypothesis that UV-induced mutations facilitate an initial immune response following ICB that results in epitope spreading and immune targeting of tumor-lineage self-antigens. Great enthusiasm exists for extending ICB to a wide range of cancer types despite early evidence showing the majority of cancer patients fail to respond to ICB. Multiple studies have also established a significant correlation between neoantigen load and ICB response. Thus the relatively low neoantigen loads present in nonmelanoma cancers pose a formidable obstacle to the broad therapeutic application of ICB. A potential method to recapitulate UV-induced neoantigens in low neoantigen tumors involves creating ?neo?- epitopes with haptens. Haptens are small molecules that conjugate to endogenous proteins to form foreign hapten-protein conjugates that can be recognized and targeted by the immune system. In Aim 2, this project will utilize mouse models of low neoantigen cancers to test the hypothesis that hapten-induced neoepitopes can overcome ICB resistance caused by neoantigen insufficiency.